1. Field of the Invention
The present invention relates generally to the fabrication of solder bumps for the interconnection of semiconductor devices, and more particularly, to under bump metallurgy (UBM) layers that have improved adhesion properties.
2. Brief Discussion of the Related Art
As semiconductor device sizes have decreased, the density of devices on a chip has increased along with the size of the chip thereby making chip bonding more challenging. One of the major problems leading to package failure as chip sizes increase is the increasing difficult problem of coefficient of thermal expansion (CTE) mismatches between materials leading to thermal and mechanical stress buildup and consequent failure. For example, in flip chip technology, chip bonding is accomplished by means of solder bumps formed on under bump metallurgy (UBM) layers overlying a chip contact pad. Frequently, improper bonding between the solder bump and the UBM layer lead to a bond that is not sufficiently strong to withstand such stresses resulting in delamination between the solder bump and the UBM layer.
The problem of delamination between the solder bump and the UBM layer is explained with reference to FIG. 1, which shows a cross-sectional view of a conventional flip chip package structure. As shown in FIG. 1, the flip chip structure includes a chip substrate 2 and a plurality of bump structures 5 (only one is shown in FIG. 1). Each bump structure 5 comprises a UBM layer 3 and a bump 1. The active surface of the substrate 2 has a first passivation layer 6 having an opening that exposes a contact pad 8. Contact pad 8 is in contact with a top metal layer 4 of substrate 2. A second passivation layer 10 is formed on contact pad 8 and first passivation layer 6.
UBM layer 3 may be multi-layered and may include layers such as an adhesion layer (not shown), barrier layer (not shown), and a wettable layer (not shown). Conventional UBM layers such as UBM layer 3 are generally flat and because they are flat, the UBM layers may easily peel off or delaminate from the contact pad 8 during thermal and mechanical stress cycles. Hence, the quality of the UBM layers is critical to the reliability and performance of the flip chip structure.
For these reasons and other reasons that will become apparent upon reading the following detailed description, there is a need for an improved bump structure in advanced IC packaging such as flip chip that avoids the reliability and performance problems associated with conventional bump structures.